The central nervous system regulates gonadotropin secretion and maintains reproductive cycles through intermittent release of luteinizing hormone-releasing hormone (LHRH). The decapeptide is synthesized in neuronal perikarya, released from neurovascular terminals into the hypophysical portal vessels and conveyed to the anterior pituitary to regulate LH secretion. The amplitude and frequency of these neuroendocrine events are influenced, in turn, by feedback actions of two circulating ovarian steroids, estrogen (E2) and progesterone (P). The objective of the proposed studies is to gain an intergrated understanding of regulatory mechanisms which govern three critical functions of LHRH neurons - LHRH synthesis, LHRH release, and LHRH actions. Three approaches have been developed towards this end: in vivo radiolabeling will be used to estimate rates of LHRH biosynthesis, pituitary microdialysis will be utilized to measure in vivo LHRH release patterns, and an in vivo isolated pituitary paradigm will be employed to analyze physiological actions of LHRH. Using these methods, we will study the physiological role(s) of LHRH neurons in the generation of the preovulatory LH surge and in homeostatic suppression of LH secretion at other times during the ovulatory cycle. One major hypothesis to be tested holds that high levels of circulating ovarian steroids directly or indirectly accelerate LHRH biosynthesis, leading to increase the amplitude of LHRH secretion and initiation of the LH surge. Possible noradrenergic and opioid peptidergic mediation of these effects will receive particular attention. LHRH biosynthesis and release will be measured during the ovulatory cycle, and the effects of E2, P, and noradrenergic or opiatergic agonists and antagonists will be assessed. Release profiles of norepinephrine throughout entire ovulatory cycles will also be characterized by microdialysis procedures. A second hypothesis holds that negative feedback actions of steroids are exerted, in part, by deceleration of a hypothalamic ultradian pulse generator. Release patterns of LHRH will be monitored in individual animals over several days following ovariectomy, with or without steroid treatments. Physiological significance of observed changes in LHRH pulse amplitude or frequency will then be assessed by mimicking these profiles in hypophysectomized rats bearing pituitary during "permissive" LHRH stimulation. These studies should provide substantial amount of new information regarding the role of LHRH neurons in the maintenance of normal ovulatory cyclicity. Such knowledge may help to enhance prospects of devising contraceptive strategies based upon perturbations of hypothalamic/hypophysical function, and aid in treatment of reproductive disorders, such as hypothalamic amenorrhea.